In Reason 8 there is a new issue with loading your own sample into NN-XT.  The issue is that when you quit Reason 8 and then re-open a file with an NN-XT device racked with your own sample, it will play the piano underneath the sample even though "solo sample" is ON.

Please see below for the solution:

1. After adding the NN-XT device,  right-click (or control+click) on the device in the rack.
2. Select "Reset Device".  You will know you have done this properly if were it normally says "piano" it now says "init patch".
3. Load your sample the usual way in the lower half of NN-XT, then click "solo sample".  SAVE!